1. Technical Field
The present invention relates to a method for manufacturing an optical fiber base material.
2. Description of the Related Art
A method for manufacturing an optical fiber base material includes producing glass fine particles through a hydrolysis of a glass material gas in a flame and depositing the produced glass fine particles onto a rotating starting member. For example, Outside Vapor Deposition (OVD) method attaches and deposits glass fine particles generated in a flame of a burner onto a starting member while the starting member is being rotated and reciprocated relative to the burner, to obtain a soot (or a deposit of glass fine particles), and then dehydrates and sinters the soot in an electric furnace.
Examined Japanese Patent Publication No. 03-009047 proposes a multi-nozzle burner that is configured such that small-diameter gas discharge ports ejecting an oxidizing gas are disposed within a combustible gas discharge port so as to surround a centrally arranged material gas discharge port. Japanese Patent Application Publications Nos. 10-101343 and 2003-226544 disclose a technique to properly adjust the focus distances of the small-diameter gas discharge ports ejecting an oxidizing gas to boost the blending of the ejected gases. Japanese Patent Application Publication No. 2006-182624 discloses that the flow rate ratio between the oxidizing gas supplied through the small-diameter gas discharge ports and the combustible gas and the flow speed ratio between the oxidizing gas supplied through the small-diameter gas discharge ports and the glass material gas may be adjusted in order to optimize the gas discharge conditions.
To fabricate a deposit of glass fine particles, a concentric multi-tube burner is used. When the concentric multi-tube burner does not sufficiently blend together a glass material gas, a combustible gas, and an oxidizing gas, however, the yield of the glass fine particles decreases. In this case, it is difficult to fabricate soot within a short period of time. On the other hand, if the blending of the glass material gas, the combustible gas, and the oxidizing gas is positively boosted, the glass fine particle production efficiency does increase but the instability of the flame increases. This lowers the ratio of the glass fine particles deposited onto the target surface. Accordingly, the improvement of the glass fine particle production efficiency is not fully turned into the increase in the deposition speed.
Furthermore, as the base materials increase in size, the flow rates of the gases supplied to the burner similarly increase. The increase in gas flow rate requires an increase in the distance between the burner and the deposition target surface for the purpose of protecting the burner against the radiation heat and preventing the glass fine particles from attaching onto the end of the burner. This requirement makes the flow of the flame generated by the burner unstable and makes it difficult to improve the deposition efficiency.